Allow UnitFactory to create units from Python. Refs #6882
Export Label unit to Python. Refs #6882

This can then be used as an arbitrary label unit on axes.
Axis::setUnit(name) now returns new unit

This avoids having to retrieve the new unit immediately after setting
it. Refs #6882
Add a test for unit replacement on an Axis from Python. Refs #6882
Merge remote branch 'origin/feature/6882_export_unitfactory'